Land-based activities constitute the largest sources of pollution in the marine environment. Marine pollution as defined by the Group of Experts on the Scientific Aspects of Marine Pollution (GESAMP), which is part of the basic framework of the UNCLOS 1982 (Article 1.4) is the:

"introduction of man, directly or indirectly, of substances or energy into the marine environment (including estuaries) resulting in such deleterious effects as harm to living resources, hazards to human health, hindrance to marine activities including fishing, impairment of quality for use of sea water, and reduction of amenities."

Under the framework of international law, sources of marine pollution are the following:

• Land-based sources and activities;
  
• Shipping and other sea-based activities such as fishing and aquaculture;
  
• Dumping;
  
• Seabed activities, both near and offshore; and
• Atmospheric sources.

Degradation of the seas and oceans, particularly, nearshore waters occurs due to the land - and sea-based activities and the concomitant pollution resulting in physical and ecological changes and/or damages such as habitat loss, altered sediment flows and atmospheric changes. Thus, mitigation not only involves cleanup responses but also the application of management measures such as closure or restriction, restoration and rehabilitation. In fisheries, for example, it is now recognized that conservation of fish habitat is an important component of building and maintaining sustainable fisheries.

A number of land-based waste products, notably nutrients and toxic pollutants, exert direct effect on economically or culturally important fish-stocks such as physiological impacts brought about by endocrine disruptors. Harmful algal blooms (HABs) are caused by microalgae, which have negative impacts on human activities, including human health, fisheries, aquaculture and tourism, among others.

In shipping, oil pollution arising from incidence of ship grounding and collision has been a major international concern. In recent years, this concern has also included hazardous and noxious substances (HNS), ballast water discharge and antifouling paints.

Anthropogenic (or “human-generated”) noise levels in the marine environment are increasing at an alarming rate. Ocean noise levels in some areas have doubled every decade for the past 60 years. There is mounting concern that noise proliferation poses a significant threat to the survival of marine mammals, fish and other ocean wildlife. Marine animals use sound to navigate find food, locate mates, avoid predators and communicate with each other. Flooding their world with intense sound interferes with these activities with serious consequences. A growing body of scientific research confirms anthropogenic noise can induce a range of adverse effects in marine mammals and other ocean creatures, from disturbance to injury and death. Sources of anthropogenic ocean noise include the use of explosives, oceanographic experiments, geophysical research, underwater construction, ship traffic, intense active sonars and air guns used for seismic surveys for oil and related activities.

To date, more than 200 international instruments, most of them drafted in the past 20 years, deal with every aspect of the environment, in particular, to protect the marine environment. In addition, a number of co-operative and collaborative mechanisms to address, manage and mitigate pollution and degradation of the environment at the global and regional levels have been developed under the auspices of the United Nations system in partnership with governments, industries, scientific institutions, international organizations, NGOs and the public at large.